Both the online industry and the ways in which people access the internet continue to evolve rapidly: Browsers are updated almost daily, and new devices are constantly being released - in 2015, there were 24,000 distinct Android devices in use around the world. Shopify, as a platform operating in that space, release new features to meet growing and evolving demands from store owners.
In response, we continuously update our themes to ensure they give the optimal performance across the range, plus we integrate new Shopify features as they are implemented on the continuously improving platform.
Generally, this might constitute a new theme version release once every couple of months, however we have had instances where one theme in particular was updated several dozen times in a single year. A version release will generally constitute us bundling together a number of (usually very minor) browser and devices issues together with some small improvement or tweak we've been wanting to make for a while - We don't generally release an update to fix a single issue (unless its a bug).
For the most part, you can adopt a similar approach to updating your theme. You do not need to be always on the latest (though some people might choose to), you can generally wait for several version updates then grab a completely fresh copy.
Updating your theme does mean you have to re-enter your theme settings and setup your homepage, however you end up with a new, better store, and going through this process offers you the perfect opportunity to refresh your storefront - and Google loves fresh content!
Not applying updates will eventually start to add up. Exactly how long your store can actually go for before it becomes 'unusable' altogether is very difficult to quantify, however its likely to be a good many years.
If you're a vested store owner wanting to keep their online presence in tip-top order, we'd certainly recommend you don't let your theme go too far over 18 months before grabbing the latest copy from the theme store and giving your shop that brand new coat of paint!
Android fragmentation: https://opensignal.com/reports/2015/08/android-fragmentation/
Shopify news blog: https://www.shopify.com/partners/blog/tagged/shopify-news
Updating your theme: https://help.shopify.com/manual/using-themes/updating-themes